In recent years, there have been proposed an aqueous coating composition for undercoating, which contains an emulsion resin (for example, refer to Patent Document 1) and a 2-liquid type aqueous coating composition for undercoating (for example, refer to Patent Document 2) from the viewpoint of environmental protection. According to these coating compositions for undercoating, a film is formed by the fusion of resin particles. Therefore, the coating compositions necessitate the use of a film-forming aid in a large amount when its resin has a high glass transition temperature. However, the use of the film-forming aid in a large amount is not favorable from the viewpoint of environmental protection, and moreover, there arises a defect such that frost damage resistance is not sufficiently imparted to the coating composition.
As an aqueous coating composition having improved frost damage resistance and improved blocking resistance, there has been proposed an aqueous resin composition in which particles having a multilayer structure are dispersed (for example, refer to Patent Document 3). However, there is a necessity in this aqueous coating composition to increase the glass transition temperature of the shell of the particles in order to impart blocking resistance to the aqueous coating composition. Therefore, the aqueous coating composition has some defects such as low film-forming property and low water permeability resistance of a coated film.
In order to improve blocking resistance, it has been proposed to use a water-soluble polymer having a high glass transition temperature and a low molecular weight, or to increase the content of a pigment in the composition. However, when the water-soluble polymer having a high glass transition temperature and a low molecular weight is used, there arises a defect such as lowering of frost damage resistance. When the content of a pigment in the composition is increased, there arises a defect such as lowering of water permeability resistance of a coated film.